Usually the guy who is dating the yandere, is really weak person and only lives because the GF spare his life (totally my generalized vision how these stories are).
But here, it's actually hinted that the Boyfriend of this Manga is potentially worse, not that he is evil or bad person.
Maybe it could be described like the scene from Crocodile Dundee, where the Yandere GF holds a knife and the BF of this story goes "That's not a knife" and pulls out a machete.